Typically, an engine of a vehicle, such as an automobile, includes a piston, and a connecting rod having a small end part coupled to the piston through a piston pin. Specifically, the piston pin is inserted through a pin insertion hole in the small end part of the connecting rod, and the small end part of the connecting rod is located on a middle portion of the piston pin in a direction along the central axis of the piston pin. Both end portions of the back face of the piston (opposite to the top face thereof) in the direction along the central axis of the piston pin form two boss portions between which the small end part of the connecting rod is interposed. The two boss portions each have a pin support hole into which both end portions of the piston pin in the direction along the central axis of the piston pin are inserted to support the end portions (see, for example, Patent Document 1).
In the engine, resonance determined by the principal structure of the engine has been known to cause combustion noise (see, for example, Non-Patent Document 1). Non-Patent Document 1 shows that engine sound has three peak frequencies of 1.7 kHz, 3.3 kHz, and 6 kHz, one of the peak frequencies (3.3 kHz) results from resonance caused by the extension and contraction of the connecting rod, and the resonance can hardly decrease in amplitude.